IPA: /ɪnˈsɛn.sɪ.tɪv.nəs/
KK: /ɪnˈsɛnsɪtɪvnəs/
The quality of not being affected by feelings or emotions; a lack of sensitivity.
His insensitiveness to her feelings made their relationship difficult.
Insensitiveness is formed from "in-" (meaning not) and "sensitivus" (from Latin, meaning sensitive) and the suffix "-ness" (meaning the state of being). The word describes the state of being not sensitive or lacking sensitivity.
Think of 'not' ('in-') being combined with 'sensitive' ('sensitivus') to remember that 'insensitiveness' means the state of lacking sensitivity.